1. Field of the Invention
The present invention relates to polishing solutions for the chemical-mechanical (chem-mech) polishing of silicon, and methods for the preparation thereof.
2. Description of the Prior Art
U.S. Pat. No. 3,436,259, Regh, et al., Polishing of Silicon by the Cupric Ion Process, Mendel and Yang, Proceedings of the IEEE, Vol. 57, No. 9, September 1969, pages 1476, et seq., and "Silicon Planar Chemical Polishing", Regh and Silvey, Electrochemical Technology, March-April 1968, pages 155, et seq., all hereby incorporated by reference, relate to a process for polishing planar silicon surfaces to a high degree of surface perfection by chemical-mechanical polishing using, e.g., an aqueous solution containing cupric ions and fluoride ions. The present invention is an improvement thereon.
During the chem-mech polishing described in Regh, et al., displacement plating of copper for the silicon on the silicon planar surface occurs with the simultaneous and continuous wiping of the silicon surface resulting in the removal of copper metal from the high areas on the silicon surface. The components most commonly used to provide such polishing solution are cupric nitrate, providing cupric ions, and ammonium fluoride solutions, providing the fluoride anions.
Such polishing solutions are prepared by mixing cupric nitrate (for example, in the form of a 100 g/l solution of CuNO.sub.3. 3H.sub.2 O) and ammonium fluoride (for example, as a 40 wt. % aqueous solution) at a 4:1 volume ratio, stirring the mixture well, allowing it to stand for 60-90 minutes and then decanting or filtering the copper-containing precipitate which forms and gradually separates from polishing solution under preparation.
The precipitate substantially reduces the cupric ion concentration in the polishing solution, generally in an amount on the order of 50 wt. %, and leads to a polishing solution which shows a lowered polishing rate. Further, the presence of solid particles in the polishing solution during polishing can cause surface damage to the silicon surface being polished.
In addition to the above defects, the separation of the precipitate, generally by the decantation or filtration, is a time consuming operation, and the disposal of the precipitate is a serious problem since the components involved are harmful to the ecology.
Attempts have been made to halt or reduce the precipitation of the copper compound, for example, acids such as nitric acid, sulfuric acid or acetic acid have been added to the polishing solution during its formation. The addition of these acids slows the precipitation, but the resultant polishing solutions are not acceptable for chem-mech polishing of a silicon planar surface because of their high acidity which produces unacceptable polished surfaces.
Other attempts to reduce the precipitation involved reducing the initial concentrations of copper nitrate and ammonium fluoride, but to obtain practically useful polishing solutions precipitate formation was still observed.
It would obviously be of substantial benefit to the art if the above defects in producing cupric cation/fluoride anion polishing solutions could be overcome.